The best bolt on Diff for the 240sx/silvia S13 chassis is the R200H differential from the S15 with the 3.6 ratio. It's a direct bolt in and allows use of the 6bolt knuckle shafts that come standard on the S13 240sx chassis, (this means no changing out to the 5bolt knuckle shafts).
I don't know much about the R34 diff and I haven't been given any specs.
But I would suggest the biggest bang for your buck swap is the S15 R200H.
For those running high boost on SR, KA, or RB it one of the better diffs to use and being that it is a Hitachi helical it's extremely reliable, unlike chinese brands such as OBX which I believe are only 3.9 and 4.1 ratios.
Hitachi Helical will save you power and fuel economy at the pump.

 

So where can I get an R200H for good price?

 

Used on ebay for $400, or new through project Nissan for $1200.

Gear ratio's for those that want to know. These Diffs show the best compatibility for drop and swaps.
S15 / MT - R200H - 3.692
S15 / AT - R200V - 3.916
S14 / KA24DE - 4.083
S14 / SR20DET - MT - R200V - 3.692
S14 / SR20DET - AT - R200V - 3.915
S13 / KA24DE - 4.083 - (Both R200 and R200V)
Z33 / MT - R200 - 3.538
Z33 / MT - R200V - 3.538
Z33 / AT - R200V - 3.357
Z32 / R200V - 4.083
J30 / R200V-3.916

Supposedly the R34 has a 3.96 gear ratio, if that is true which I don't know for sure it wont pose for better gas mileage than a s15R200H.

 

BUMP...
Courtesy Nissan has the HLSD for $1000 brand new.
http://www.courtesyparts.com/Merchan..._Code=S15-HLSD

 

I am really curious about people getting 30+ MPG. What did you do to your car? btw. my car is completely stock and I get around 26/27 MPG

 

High flow CAT, AEM intake, FAL electric fans, and aluminum fly wheel was when I noticed the most significant increase in gas mileage (when not using the SAFC).
Enjuku racing sells an aluminum drive shaft that I will probably add next year it cost $500, in the long run I'll save more money in gas at the pump with it.
Dropping over weight inertia from parts of the rotating assembly's of the engine drops LOAD requirements, and as a trade off the engine runs more efficiently using less fuel under LOAD requirements. This is because it takes less energy to increase momentum which pulls the vehicle inches sooner on less fuel. It's simple physics.
So with lighter drive train parts you get the best of both worlds better fuel economy, as well as performance enhancement.

 

hmm how about underdrive crank pully? has anyone try it? does the ebay 40.00 one work fine? or should I stay away from them?

 

Under drive crank pulley's wont hurt a low rev engine like the KA. The only problem is that the timing marks on the pulley are off. I bought one off of ebay and I don't use it because the timing marks are off. Stillracing is preparing a whole underdrive pulley set for me for $200, it uses the alternator, crank, and power steering pulleys.

 

Stillracing or Stillen Racing?

Underdrive pulley kits do not increase hp, just eleviates some of the stress of the engine from working harder to run things like your AC etc.

 

Aluminum flywheels definitely work though. The weight comparison between a flywheel and a crank pulley are tremendous.Crank pulleys only serve to under drive and shrink it's rotation to pulling the belts. Degradation loss caused from the belts is reduced from the shrunken diameters of the pulleys, but this means that the alternator, and power steering pumps need to be reduced as well or the light weight pulley that would be installed to the crank would make the engine work harder, (it's basically shrinking it's pulling ratio, and shorter length of belts). Just shrinking the crank pulley shortens the belts which also uses less energy than a longer belt would, so I think there wouldn't be any gains or losses to just using a crank pulley.


The company is called Stillracing

 

so Stillracing makes a full underdrive kit for KA? If so, Im buying a set! Especially for 200?

 

so what about an aluminum drive line, anyone care to chime in and let us know if they are worth the money?

 

For our normal applications, aluminum drive shafts would reduce the load needed to make each revolution. Less load, means engine doesnt work as hard to make the car go. Sorry, brain dead at the moment.

Carbon Fiber drive shafts... another story. Hear bad things about them. But the 350Z and G35 uses carbon fiber shafts and they work fine. Depends on who you get your CF shafts from.

 

Yes they do P.M me or search for it in NICO, there not one of our vendors.
Carbon fiber has very little tensile strength compared to aluminum or chromolly steel. Thats why it has many problems when used as a drive shaft.

 

Dude Where Are You People Getting The Mileages From I Only Average 23 City/highway